posix-timers: Fix inverted SIGEV_NONE logic in common_timer_get()

The refactoring of the posix-timer core to allow better code sharing
introduced inverted logic vs. SIGEV_NONE timers in common_timer_get().

That causes hrtimer_forward() to be called on active timers, which
rightfully triggers the warning hrtimer_forward().

Make sig_none what it says: signal mode == SIGEV_NONE.

Fixes: 91d57bae ("posix-timers: Make use of forward/remaining callbacks")
Reported-by: Ye Xiaolong <xiaolong.ye@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170609104457.GA39907@inn.lkp.intel.com